The White House orchestrated a simulated cyberattack on New York City’s power supply during a summer heat wave late Wednesday to illustrate not only potential human and economic casualties, but to tee up support for Senate passage of a sweeping cybersecurity bill.

During a classified briefing in the Office of Senate Security, Homeland Security Secretary Janet Napolitano and White House counterterrorism adviser John Brennan showed lawmakers how a hacker could breach control systems of the city’s electric system and trigger a ripple effect throughout the population and private sector, according to a source familiar with the scenario.

“The fact that we could be subject to a catastrophic attack under the right circumstances and we now know some of the things that would help us to protect against such an attack, that’s why it’s important now for the Congress to take this up,” Napolitano said in an interview with POLITICO.

Such an attack could lead to mass casualties or strike a devastating blow to the U.S. economy, Napolitano said.

Yet the aim of the cyber exercise wasn’t just to scare senators straight, but also build support for the Senate to act on a sweeping cybersecurity bill that includes government requirements on private sector operators of the nation’s “critical infrastructure” — such as power grids and water supplies.

The bill, backed by Sens. Joe Lieberman (I-Conn.), Susan Collins (R-Maine), Jay Rockefeller (D-W.Va.) and Dianne Feinstein (D-Calif.), has been challenged in recent days by a rival plan that leaves out industry mandates.

During the simulation, the hacker gains access to the electric supply’s control system through a simple “spearphishing” attack, in which a worker merely clicks on a link in an email that appears to be from someone they know.

A who’s who of law enforcement and Defense officials also participated, including FBI Director Robert Mueller, National Security Agency Director Gen. Keith Alexander and Chairman of the Joint Chiefs of Staff Martin Dempsey.

Noting the prominent lineup of officials at the briefing, Napolitano said, “I think the message given out was: This could happen now.”

When asked if the cyberattack scenario described to the Senate has happened in U.S. before, Napolitano bluntly answered: “Not in that way.”

But she noted that the threat is real and growing.

Last year, DHS responded to more than 106,000 incidents reported across the country, which was “significantly higher” than the previous year, Napolitano said.

“The sophistication has really increased over my three-plus years as secretary,” Napolitano said of the cyberthreats.

There are two competing cybersecurity bills on the table in the Senate.

Backed by the White House and Senate Democrats, the Lieberman bill would require operators of critical infrastructure to work with DHS to set security standards they must follow. The U.S. Chamber of Commerce and other business groups have raised concerns that this approach is too regulatory and would refocus industry’s attention from security to complying with new government mandates.

By contrast, the bill offered by Sen. John McCain (R-Ariz.) and seven other Republicans attempts to avoid new regulations on industry and focuses on fostering information sharing about cyberthreats between the government and companies.

Napolitano endorsed the approach taken in the Lieberman measure.

“We’re talking about the covered core infrastructure where an attack could have a huge impact on the economy, could have catastrophic loss of life [or] personal displacement,” she said. “If you’re in that category, we believe that there should be standards that you need to follow in order to believe the country is protected.”

But some Senate Republicans and industry members have questioned whether DHS is ready to take on this elevated cybersecurity role.

Napolitano has been taking steps in recent weeks to convince lawmakers and other government officials that DHS is quite capable of coordinating the nation’s cyber response. Undeterred by the doubters, Napolitano noted that the department already has “very, very broad and extensive experience in the cyber arena.”

“I think many of those comments are made by those who don’t yet know the full extent of what we already are doing and the fact that we already have the ability to respond to more than 106,000 incidents last year,” the secretary said.

The administration has rallied around the Lieberman bill. A couple weeks before the bill was introduced, the administration led another classified briefing for senators to demonstrate why cybersecurity legislation was needed. President Barack Obama also phoned Lieberman last week to voice the administration’s support for the measure.

“It just shows we need to further educate members about what’s going on, and the purpose of the bill and the reasons why the administration is so strongly behind it,” Napolitano added. “It’s time to take this up in statute, and get the private sector up to a basic level of security and improve the level of information sharing so it is timely and complete.”

Presidential election year politics could jeopardize either Senate bill from moving forward. But Napolitano said there was a "sense of commonality in the room" among senators that action needs to be taken now before it's too late.

The cyber exercise drew more than 30 senators. Several senators said the briefing only reinforced their desire to address the security issue, but they voiced varying levels of support for the Lieberman measure.

Sen. Olympia Snowe (R-Maine), who is seen as a key Republican vote for the Lieberman bill, said after the briefing that she has not made a decision on which bill to support.

“I still have issues that I wanted to address,” Snowe said when asked why she did not sign onto to co-sponsor the Lieberman bill. “I just think we need to have an open, unfettered debate on the floor of the Senate to work through these issues.”

When leaving the simulation, Sen. Kay Bailey Hutchison (R-Texas) said there isn't a disagreement that cybersecurity is an important defense issue. The difference in opinions between the Republicans and the administration still lies in how to tackle the problem.

"They're trying to say that we have to have DHS determine critical infrastructure and then set regulations in place and I think that's where we differ," said Hutchison, who is co-sponsoring the rival GOP bill. "I think we can produce a lot in information sharing with liability protections and antitrust protections, which are not in the administration's bill."

This article first appeared on POLITICO Pro at 7:21 p.m. on March 8, 2012.